1. Field of the Invention:
The present invention relates generally to heat exchange systems, and in particular, to a gas-to-water heat recovery system which utilizes an array of heat pipes for collecting heat from a stream of heated gas and transferring the heat into a volume of water for the production of steam.
2. Description of the Prior Art:
Heat recovery from industrial waste gas sources presents an ever increasing opportunity for economical operation of thermal systems. The economic advantage from any form of heat recovery depends upon the availability and cost of fuels. Obviously, savings from heat recovery increase as fuel costs rise. As the cost of energy constantly increases, different types of systems and methods are being devised to recover and transfer thermal energy which would otherwise be lost.
Conventional heat exchange apparatus operates in several heat recovery modes including air to gas, gas to water, and gas to organic fluids. The selection of the mode of heat recovery depends upon the characteristics of the application, the processes used by the particular industrial facility, and the economic need for a given service. For example, steam can be generated at low pressure for heating or absorption air conditioning applications, at medium pressures for processing, or at higher pressures with or without superheat for electrical power generation.
The recovery of heat energy by the generation of either high pressure or low pressure steam is probably the most common means of fuel and energy conservation because steam carries tremendous heat energy per unit weight, consisting of sensible and latent heat. Various types of heat recovery boilers are available for the recovery of heat energy by the generation of either high pressure or low pressure steam. Examples of conventional heat recovery boilers include units of straight tube banks attached to fixed or floating headers and units of serpentine (return bend) elements. The circular coil type and the horizotal serpentine element type require forced recirculation. Vertical tube units may operate in either forced or natural circulation modes. Larger low pressure heat recovery applications usually employ the natural circulation system, commonly of the two drum variety. This is essentially a closed loop system, with one leg of the loop receiving heat and the other serving as the downcomer that does not receive heat. The difference in water and steam specific volume in the heated portion relative to the water only portion in the downcomer creates a natural upflow of water and steam from the lower to the upper drum. Steam is released in the upper drum, passing through a scrubbing section for moisture removal. A given volume of water may circulate several times between the drums before it receives enough enthalpy to change phase to steam.
The multiple drum natural circulation boiler is stable, the boiling heat transfer coefficients are high, and the system has reserve water for variable demand. Among the several disadvantages inherent in this arrangement are an excessive number of tubes required for the downcomers, bent tubes are required in order to accommodate differential expansion, and the boiler drums must be thick walled for high pressure operation.
In the operation of conventional waste heat boilers, the rate of heat transfer from waste gases to the boiler water depends upon the temperature and specific heat of the gases, the velocity and direction of the gases over the heat absorbing surfaces of the boiler, and the cleanliness of the surfaces. For proper heat transfer from the waste gases to the boiler water there must be sufficient stack or an induced draft fan to overcome the draft losses due to the required flow of the gases over the heat absorbing surfaces with an allowance for fouling of these surfaces. Compared with direct firing arrangements, the gas temperatures are generally lower and consequently the radiation component in the heat transfer mechanism is also lower. Therefore, the tendency with waste heat boilers is to design for higher gas velocity over the tubes in order to increase the convection component of heat transfer. However, a significant number of industrial processes generate a substantial amount of heated waste gas which is only available for recovery of thermal energy at relatively low flow velocities. Consequently, there exist a number of industrial processes in which recovery of waste heat by conventional heat exchangers is relatively inefficient because of the low flow velocities involved. In view of the constantly increasing cost of energy, there is a continuing need for new and improved systems for recovering waste heat which operate effectively at relatively low flow velocities.